Various types of cordless telephone systems are described, for example, in U.S. Pat. No. 4,291,197. In such systems, the remote station is coupled to the base station by a wireless link. The base station is connected to the telephone line, and it responds to ring signals and incoming calls from remote callers to ring the remote station and to enable a twoway conversation to be carried out between the person at the remote station and each caller.
In the usual present-day type of cordless telephone system, the audio and other signals are frequency modulated, on the carrier, and they have a relatively narrow bandwidth. The usual range of present-day cordless telephone systems is about 500 feet.
The narrow band transmission characteristic of the present-day cordless telephone system has problems in that the transmitted signals have peaks and nulls as the telephone at the remote station is moved from place-to-place. This condition is alleviated in the embodiment of the present invention to be described by providing multiple communication channels between the base station and the remote station, and if communication fails in any channel to select automatically another channel.
Other problems encountered in present-day cordless telephone systems include inadvertent answering of the neighbors' telephone when two or more systems are in the same vicinity, and the ability to eavesdrop on the neighbors' conversations. These latter problems are solved in the system of the present invention by digitizing and encoding the ring signals, the dialing pulses and the audio signals. This encoding is preferably in accordance with a pseudo random code to facilitate synchronization between the base station and the remote station.